Long Answer: How low are you? If it is low enough that the alcohol content would be sufficient, I wouldn't do it and just take rigourous notes of what you did to miss the gravity so when you do another batch you can audit your process.

ALSO!!! Did you temperature adjust for your hydrometer reading? This is very important as if your wort is above the temp the hydrometer states on it, it will read low.

There are various online calculators for adjusting for temperature. I use Dave's Dreaded which seems to be pretty close.

I have no idea how this happened because my other two beers have been fine but I managed to get 1.035 on an IPA that was supposed to be near 1.07. Adjusting for temperature MAYBE the beer was 1.04. I also would like any advice anyone has as to why this happened. I was making a small batch (1.5 gal) on the stove top in a grain bag using amounts of water given to me by hopville. And I mashed for 70 minutes trying to stay around 151. There was a good amount of fluctuation so maybe that had to do with it? I don't really know and I'm very confused

If you have not pitched yet, the best option is to boil some DME for at least 15 minutes, cool it to yeast temp, add it to your fermenter, give it a vigorous shake, and then pitch the yeast. If you can fit the extra volume in your fermenter, using a rough estimate I would do about 1 lbs of DME (2.75 cups) in a half gallon of water. This should give you (again roughly) 43 gravity points. Thus putting you in the 1.077-81 range. maybe try 2.25 or 2.5 cups to come in a little lower. It will likely change the color of the beer and maybe the taste, but it will bring the gravity up.